1. Field of the Invention
The present invention relates generally to a feedforward linear amplifier for increasing linearity of a high power amplifier used in a radio communication system, and more particularly, to an optimal control method for an adaptive feedforward linear amplifier, whereby an adaptive controller can calculate an optimal control voltage by using an adaptive algorithm for the linear amplifier, providing for changes in environment.
2. Description of the Related Art
A feedforward linear amplifier includes two circuits and receives at least one carrier input signal at the predetermined range of frequency. The input signal is applied to the first circuit inclusive of a main power amplifier, in which case a distortion signal occurs. In the meantime, only the distortion signal component generated from the main power amplifier can be extracted by properly controlling variable attenuator and variable phase shifter in the first circuit. The distortion signal component thus extracted is transmitted to the second circuit. The second circuit amplifies the distortion signal component with an attenuation and phase shift such that the distortion signal component of the main power amplifier is removed from the final output terminal. In the present invention, emphasis is laid on the procedure of controlling the two variable attenuators and the variable phase shifter using the controller.
To read an output of the first circuit, the related art controller determines a desired frequency band, operates a voltage-controlled oscillator (VCO) at the frequency band and determines the position of a radio frequency (RF) switch. In order to minimize the signal component read out from the first circuit, a power detector detects the strength of the main signal component and after changing the phase shifter control voltage, the main signal strength is detected again. If the main signal strength is decreased, the phase shifter control voltage is changed in the opposite direction; and if the main signal strength is decreased, the phase shifter control voltage is changed in the same direction. Iterative procedures are performed N-M times for variations of the phase shifter and M times for the control of the variable attenuators. These procedures are iteratively performed for the phase shifter and the variable attenuator until the main signal strength is lower than a threshold. With the main signal strength lower than the predetermined level, then the second circuit is controlled. The procedures for the second circuit are analogous to those for the first feedback with an exception that the controller controls the distortion signal strength to be minimized.
There exist different methods to operate the controller appropriately, such as pilot signal method, vector modulator method, non-feedback loop method, loop method, trial-and-error method and a combination of them.
Among these methods, the trial-and-error method that is similar to the present invention requires more repetitions for calculating an optimal control voltage and the voltage value is controlled in the fixed step-size only. Therefore, different program steps are needed, resulting in retarded adaptation to changes in environment. It is further another disadvantage of the related art control method that the variation of the control voltage cannot be minimized sufficiently when the control voltage is close to the optimal value.